(i) Field of the Invention
The present invention relates to a process for preparing hydroxybenzoic acids. More specifically, it relates to an industrially advantageous process for preparing alkylsalicylic acids such as a 3,5-dialkylsalicylic acid which are useful as synthetic materials of chemicals such as developers for pressure-sensitive recording papers, agricultural chemicals and antioxidants.
(ii) Description of the Prior Art
As a process for preparing a hydroxybenzoic acid, there is known the Kolbe-Schmidt reaction from old days which comprises reacting an alkali metal salt of a phenol with carbon dioxide.
In recent years, as improved techniques of the Kolbe-Schmidt reaction, methods in which the reaction proceeds in the state of a solution or a slurry instead of a solid phase reaction have intensively been researched from the viewpoint of some industrial advantages.
For example, Japanese Patent Application Laid-open No. 165341/1988 discloses a method for preparing a 3,5-dialkylsalicylic acid which comprises adding an aqueous alkali metal hydroxide solution and a 2,4-dialkylphenol to a hydrocarbon solvent, the amount of the 2,4-dialkylphenol being in excess of the alkali, removing water by azeotropic dehydration to synthesize an anhydrous alkali metal salt of the 2,4-dialkylphenol, and reacting this salt with carbon dioxide. In this method, however, the reaction mass tends to form a paste, which makes stirring difficult. In consequence, as shown in comparative examples which will be described hereinafter, a sufficient reaction yield cannot be obtained.
Japanese Patent Application Laid-open No. 34944/1989 discloses a reaction in a mixed solvent of a hydrocarbon solvent such as toluene and sulfolane, but sulfolane is a viscous and high-boiling solvent; and hence, because of the adhesion of sulfolane to crystals, it is difficult to recover all of the sulfolane.
In a method disclosed in Japanese Patent,Application Laid-open No. 178947/1991, a 2,4-dialkylphenol is reacted with an alkali metal hydroxide in a lower alcohol, and the lower alcohol and produced water are then distilled off. The resulting anhydrous alkali metal salt of the 2,4-dialkylphenol is next reacted with carbon dioxide. However, the reaction with carbon dioxide is carried out in a non-solvent solid phase state, which is not considered to be an industrially advantageous method.
In Japanese Patent Application Laid-open No. 90047/1991, there is disclosed a method which comprises heating a 2,4-dialkylphenol and an alkali metal hydroxide in a mixed solvent of a hydrocarbon solvent and 1,3-dimethyl-2-imidizolidinone, carrying out azeotropic dehydration to form an anhydrous alkali metal salt of the 2,4-dialkylphenol, reacting the same with carbon dioxide in the mixed solvent to obtain a 3,5-dialkylsalicylic acid. In this method, the reaction solution is directly discharged into an acidic liquid to take out the product. However, it is difficult to recover expensive 1,3-dimethyl-2-imidazolidinone from the aqueous layer, and so this method is not considered to be an industrially advantageous method. In addition, it is described in the disclosed specification that the amount of 1,3-dimethyl-2-imidazolidinone to be used is preferably in the range of 1 to 5 wt % based on the weight of the raw material phenol from an economical viewpoint, but when the compound is used in such an amount, the reaction system becomes a paste at the time of the dehydration, so that the reaction system is substantially close to the state of a solid reaction and hence stirring by a conventional of stirrer is impossible. Even if the reaction is forcedly continued, a sufficient reaction yield cannot be obtained.
In the case that the reaction is carried out in an aprotic polar organic solvent such as sulfolane or 1,3-dimethyl-2-imidazolidinone, the high reaction yield can be obtained, but as described above, there are large problems regarding the recovery of the product from the reaction solution and the recovery of the solvent.
That is to say, in the case that the aprotic copolar organic solvent is used as the reaction solvent, the reaction proceeds to a the high reaction yield, but o after the reaction, even if it is attempted that the alkali metal salt of the 3,5-dialkylsalicylic acid is crystallized and collected from the reaction solution, the recovery yield is much lower as compared with the reaction yield, because the solubility of this metal salt in the aprotic polar organic solvent is high. It is also possible that a large amount of a poor solvent can be added to recover the product, but the volume efficiency is very poor. Furthermore, a wet type of the obtained alkali metal salt of the 3,5-dialkylsalicylic acid contains a large amount of the aprotic polar organic solvent. Since a certain interaction is present between the alkali metal salt of the 3,5-dialkylsalicylic acid and the aprotic polar organic solvent, the removal of the aprotic polar solvent by washing with the poor solvent is difficult. If the thus obtained alkali metal salt of the 3,5-dialkylsalicylic acid containing the aprotic polar organic solvent is dissolved in water to do acidifying-out, all of the contained aprotic organic solvent transfers to the crystallization by acidification filtrate and it is consequently lost.
As a technique other than the above crystallization for recovering the alkali metal salt of the 3,5-dialkylsalicylic acid, there is a method which comprises concentrating the reaction solution. In this method, however, the distillation efficiency of a high-boiling solvent such as 1,3-dimethyl-2-imidazolidinone or sulfolane is poor, and as described above, a certain interaction is present between the alkali metal salt of the 3,5-dialkylsalicylic acid and the aprotic polar organic solvent, so that the recovery of its total amount by the distillation is impossible.
On the other hand, even in the case that the reaction solution is directly dissolved in water with-out the alkali metal salt of the 3,5-dialkyl-salicylic acid and then separated, wherein the separated aqueous layer is further subjected to the acidifying-out, almost all aprotic polar organic solvent transfers to the acidifying-out filtrate and is finally lost. In order to recover the aprotic polar organic solvent in a large amount of water, it is necessary to distill a large amount of water, and for this reason, the technique is not industrially applicable in view of an energy efficiency. Thus, on the basis of the conception that the loss of the aprotic polar organic solvent is unavoidable, it can also be considered to reduce the amount of the aprotic polar organic solvent to be used from an economical viewpoint, but if the amount of the aprotic polar organic solvent is reduced, the reaction solution correspondingly becomes a substantially solid phase reaction in a paste state, so that the reaction yield also deteriorates.
In consequence, an object of the present invention is to provide an industrially applicable process for preparing a hydroxybenzoic acid from a phenol in accordance with the Kolbe-Schmidt reaction by the use of an aprotic polar organic solvent as a reaction solvent, and this process is excellent in reaction yield and product recovery yield and can substantially completely recover the used aprotic polar organic solvent.
The present inventors have intensively investigated with the intention of solving the problems of the conventional technique, and as a result, it has been found that in a process which comprises reacting a phenol with an alkali metal compound by the use of an aprotic polar organic solvent as a reaction solvent to form an alkali metal salt of the phenol, and then reacting this alkali metal salt with carbon dioxide to obtain a hydroxybenzoic acid, an alkali metal salt of the hydroxybenzoic acid can be quantitatively recovered by sufficiently raising the ratio of the phenol to the hydroxybenzoic acid and the aprotic polar organic solvent even after the reaction, and the obtained wet product does not contain any aprotic polar organic solvent. In consequence, the present invention has been completed.
That is to say, the present invention is directed to a process for preparing a hydroxybenzoic acid which comprises reacting a phenol with an alkali metal compound by the use of an aprotic polar organic solvent as a reaction solvent to form an alkali metal salt of the phenol, and then reacting this alkali metal salt with carbon dioxide to obtain the hydroxybenzoic acid, said process comprising the step of carrying out the reaction under conditions that a molar ratio of the phenol to the total of the alkali metal compound and the aprotic polar organic solvent is larger than 1; and further the steps of precipitating crystals from the reaction solution, separating the solid from the solution to obtain the wet alkali metal salt of the hydroxybenzoic acid, dissolving the wet alkali metal salt in water, and acidifying-out the solution to obtain the hydroxybenzoic acid.
A In the present invention, an alkali metal compound and a phenol in an amount in excess of the alkali metal compound are heated in an aprotic polar organic solvent to form an alkali metal salt of the phenol. At this time, produced water is removed from the system, and after the completion of the dehydration, the obtained alkali metal salt of the phenol is then reacted with carbon dioxide to obtain a desired hydroxybenzoic acid.
No particular restriction is put on a kind of phenol which can be used in the present invention, and phenol and optionally substituted phenols can be used. Examples of the substituents include straight-chain and branched alkyl groups having 1 to 20 carbon atoms, alkenyl groups, alkoxy groups, acyl groups; a phenyl group, an amino group, a carboxyl group, a sulfonic group, thiol groups and a nitro group, and these groups may optionally be substituted. In addition, the number of the substituents is optional, and no particular restriction is put on a substituted position. In the case that the phenol has a plurality of substituents, these substituents may be the same or different.
Among these substituents, alkyl substituted phenols and alkoxy substituted phenols which possess a high reaction selectivity and a high reaction yield are preferable, and above all, dialkylphenols are preferable. Examples of the alkyl groups include methyl, ethyl, n-propyl, i-propyl, n-butyl, sec-butyl, tert-butyl, octyl and 2-ethylhexyl.
No particular restriction is put on the alkali metal compound which can be used in the present invention, and alkali metal hydroxides and alkali metal alcoholates can be used. From the viewpoints of the ease of handling and economy, however, alkali metal hydroxides typified by sodium hydroxide and potassium hydroxide can be used. They can be used in the state of a solid or an aqueous solution having an optional concentration.
The amount of the phenol for use in the present invention should be regulated so as to be 1 to 9 mols, preferably 2 to 6 mols per mol of the alkali metal salt of the produced hydroxybenzoic acid even after the completion of the reaction in addition to more than a mol equal to the aprotic polar organic solvent. If the ratio of the phenol to the alkali metal salt of the hydroxybenzoic acid is more than the above range, volume efficiency decreases. On the other hand, if the ratio of the phenol to the alkali metal salt of the hydroxybenzoic acid and the aprotic polar organic solvent is less than the above level, crystallization yield of the alkali metal salt of the hydroxybenzoic acid obtained by the reaction deteriorates, and in addition, the wet alkali metal salt of the hydroxybenzoic acid contains a large amount of the aprotic polar organic solvent. Therefore, when the wet product is dissolved in water and then separated, the total amount of the contained aprotic polar organic solvent transfers to the crystallization by acidification filtrate of the separated aqueous layer and it is finally lost. The above regulation of the amount of phenol may be carried out at the time of the feed of the phenol and the alkali metal compound for the reaction, before the reaction of the alkali metal salt of the phenol with carbon dioxide, during the reaction, or after the completion of the reaction. However, in the case that the regulation is done before the reaction of the alkali metal salt of the phenol with carbon dioxide, during the reaction, or after the completion of the reaction, it is necessary that the excess phenol should be removed from the recovered solution after the crystallization and the separation of the alkali metal salt of the hydroxybenzoic acid, and then be returned to a raw material system for the next reaction, which makes the efficiency low. Accordingly, it is most preferable that the regulation is carried out at the time of the feed of the phenol and the alkali metal compound for the reaction.
In the present invention, the phenol is preferably fed so as to be in the range of 2 to 10 mols per mol of the alkali metal compound and, in addition, so as to be 2 mols to 30 mols per mol of the aprotic polar organic solvent. As described above, if the amount of the phenol is more than this range, the volume efficiency decreases. On the other hand, if the ratio of the phenol is less than the above range, the crystallization yield of the alkali metal salt of the hydroxybenzoic acid obtained by the reaction deteriorates, and in addition, the wet alkali metal salt of the hydroxybenzoic acid contains a large amount of the aprotic polar organic solvent. Therefore, when the wet product is dissolved in water and then separated, the total. amount of the contained aprotic polar organic solvent transfers to the acidifying-out filtrate of the separated aqueous layer and it is finally lost.
Examples of the aprotic polar organic solvent which can be used in the present invention include amide solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, 1,3-dimethyl-2-imidazolidinone, 1,3-dipropyl-2-imidazolidinone and 1,3-dibutyl-2-imidazolidinone, and sulfur-containing solvents such as dimethyl sulfoxide and sulfolane. These may be used singly or in a combination of two or more thereof. The organic solvent which is excellent in stability in the presence of the alkali metal compound is preferable, in particular, the employment of 1,3-dimethyl-2-imidazolidinone or sulfolane is preferable. The amount of the aprotic polar organic solvent to be used depends on the amount of the phenol to be used, but it is in the range of 0.3 to 3 mols, preferably 0.3 to 1.5 mols per mol of the alkali metal compound. If the amount of the aprotic polar organic solvent is less than the above range, the reaction system forms such a paste so that stirring is difficult by a conventional stirrer, and in addition, the sufficient reaction yield cannot be obtained. On the other hand, if the amount is more than the above range, the crystallization yield of the alkali metal salt of the hydroxybenzoic acid produced by the reaction deteriorates.
In the present invention, water produced during the formation of the alkali metal salt of the phenol from the phenol and the alkali metal compound is removed from the system under atmospheric pressure or reduced pressure, together with water which is accompanied by the raw materials for the reaction. In order to effectively carry out the dehydration, an azeotropic dehydrator may be used. No particular restriction is put on a kind of azeotropic dehydrator, but a hydrocarbon solvent is usually used. Examples of the hydrocarbon solvent include aliphatic hydrocarbons such as hexane, heptane, octane, nonane, decane, undecane, dodecane, ligroin and kerosene, aromatic hydrocarbons such as benzene, toluene, xylene, mesitylene, ethylbenzene, cumene, diphenyl ether and naphthalene, and halogenated hydrocarbons such as 1,2-dichloroethane, 1,1,2-trichloroethane, chlorobenzene, o-dichlorobenzene and p-dichlorobenzene. These can be used singly or in a combination of two or more thereof. The amount of the azeotropic dehydrator to be used depends on the amount of water entrained into the system, but it is usually 2 to 10 times by weight as much as the amount of the water in the system. After the completion of the dehydration, the used azeotropic dehydrator may be distilled off from the system or it may remain in the system and then forwarded to the next reaction.
In the present invention, the reaction by which the alkali metal salt of the phenol is produced from the phenol and the alkali metal compound is carried out under atmospheric pressure or reduced pressure. At this time, a reaction temperature depends on a kind of selected azeotropic dehydrator and a vacuum degree, but water distilled by heating the reaction mixture up to an azeotropic temperature of the azeotropic dehydrator and water under this vacuum degree is removed from the system. At this time, the azeotropic dehydrator distilled off together with water may be returned to the system as it is, or it may freshly be added as much as distilled off.
No particular restriction is put on the reaction of the thus obtained alkali metal salt of the phenol and carbon dioxide, but it is usually carried out in an autoclave at a reaction temperature of 80 to 200xc2x0 C. under a carbon dioxide gas pressure of 1 to 20 kg/cm2.
The reaction time depends on the reaction temperature and the carbon dioxide gas pressure, but usually, a time of about 1 to 6 hours is enough. Depending on the kind of raw material phenol or produced hydroxybenzoic acid, the reaction system becomes a paste so that stirring is difficult sometimes after the production of the hydroxybenzoic acid. In this case, a lubricant which is inert to the reaction may be previously added and the reaction with carbon dioxide may be then carried out, which does not provide any influence on the reaction yield. No particular restriction is put on the kind of lubricant, but a hydrocarbon solvent is usually used in consideration of a reaction morphology and handling ease. Examples of the hydrocarbon solvent include aliphatic hydrocarbons such as hexane, heptane, octane, nonane, decane, undecane, dodecane, ligroin and kerosene, aromatic hydrocarbons such as benzene, toluene, xylene, mesitylene, ethylbenzene, cumene, diphenyl ether and naphtha lene, and halogenated hydrocarbons such as 1,2-dichloroethane, 1,1,2-trichloroethane, chlorobenzene, o-dichlorobenzene and p-dichlorobenzene.
The thus obtained alkali metal salt of the hydroxybenzoic acid is crystallized by a means such as cold crystallization or reprecipitation by the use of a poor solvent, and then isolated by a conventional solid-liquid separating operation such as filtration or centrifugal separation. Examples of the poor solvent which can be used include hydrocarbon solvents, and typical examples of the hydrocarbon solvents include aliphatic hydrocarbons such as hexane, heptane, octane, nonane, decane, undecane, dodecane, ligroin and kerosene, aromatic hydrocarbons such as benzene, toluene, xylene, mesitylene, ethylbenzene, cumene, diphenyl ether, and naphthalene, and halogenated hydrocarbons such as 1,2-dichloroethane, 1,1,2-trichloroethane, chlorobenzene, o-dichlorobenzene and p-dichlorobenzene, and these can be used singly or in a combination of two or more thereof. The hydrocarbon solvent which can be used as the poor solvent may be the same as or different from the hydrocarbon solvent usable as the above azeotropic dehydrator and lubricant, but it is preferable to use the same solvent in consideration of the recovery of the solvent.
The obtained wet alkali metal salt of the hydroxybenzoic acid is dissolved in water, and the organic layer mainly comprising the used hydrocarbon solvent is separated and removed. The resulting aqueous layer is neutralized with a mineral acid such as hydrochloric acid, sulfuric acid and nitric acid, and the precipitated crystals are isolated by a solid-liquid separating operation such as filtration and centrifugal separation to obtain the substantially impurities-free hydroxybenzoic acid. When the washing of the wet alkali metal salt of the hydroxybenzoic acid is insufficient, the aqueous layer obtained by the separation contains the phenol. In this case, the same hydrocarbon solvent is added and extraction/separation is done so that the phenol may transfer into the organic layer, and the resulting aqueous layer is then neutralized with a mineral acid such as hydrochloric acid, sulfuric acid and nitric acid, and the precipitated crystals are isolated by a solid-liquid separating operation such as filtration and centrifugal separation to obtain the substantially impurities-free hydroxybenzoic acid.
In the present invention, the recovered solution from which the alkali metal salt of the hydroxybenzoic acid has been separated substantially comprises the phenol, the aprotic polar organic solvent and the hydrocarbon solvent used as the azeotropic dehydrator and as the lubricant. Thus, after the hydrocarbon solvent has been recovered by an operation such as distillation, the recovered solution is returned to the raw material system, and the alkali metal compound and the phenol are added as much as their consumed amounts to rebuild the reaction recycle system. In a certain case, the recovered solution contains small amounts of the alkali metal salt of the hydroxybenzoic acid and inorganic salts, but in such a case, the recovered solution is washed with a small amount of water and extracted so that the alkali metal salt of the hydroxybenzoic acid and the inorganic salts may transfer into the aqueous layer. Afterward, the hydrocarbon solvent is recovered by an operation such as distillation, and the recovered solution is returned to the raw material system, and the alkali metal compound and the phenol are added as much as their consumed amounts to rebuild the reaction recycle system. Moreover, the recovered hydrocarbon solvent can be used again as the azeotropic dehydrator and the lubricant.
As described above, according to the present invention, there can be provided an industrially applicable process for preparing a hydroxybenzoic acid from a phenol in accordance with the Kolbe-Schmidt reaction by the use of an aprotic polar organic solvent as a reaction solvent. This process is excellent in a reaction yield and a product take out yield and can substantially completely recycle the used aprotic polar organic solvent.